Versatile System for Dual Carrier Transformation in Orthogonal Frequency Division Multiplexing

ABSTRACT

The present invention provides a versatile system for selectively spreading carrier data across multiple carrier paths within an Orthogonal Frequency Division Multiplexing (OFDM) system ( 200 ), particularly an ultra-wideband (UWB) system. The present invention provides a data input ( 202 ), which passes data to a randomizer ( 204 ). The data then passes to a convolutional code function ( 206 ), the output of which is punctured by puncturing function ( 208 ). An interleaver function ( 210 ) receives the punctured code data, and cooperatively operates with a mapper element ( 218 ) to prepare the coded data for pre-transmission conversion by an IFFT ( 220 ). The mapper element ( 218 ) comprises a dual carrier modulation function ( 216 ), which associates and transforms two punctured code data elements into a format for transmission on two separate signal tones.

PRIORITY CLAIM

This application is a divisional of U.S. application Ser. No. 11/099,317filed Apr. 5, 2005 which claims priority to U.S. Provisional ApplicationNo. 60/559,845, filed Apr. 6, 2004. This application hasinventors-in-common and is assigned to Texas InstrumentsIncorporated—assignee of U.S. patent application Ser. No. 10/688,169.

TECHNICAL FIELD OF THE INVENTION

The present invention relates generally to the field of wirelesscommunications and, more particularly, to structures and methods formaximizing utilization and efficiency of communications utilizingorthogonal frequency division multiplexing through frequency domaintransformations.

BACKGROUND OF THE INVENTION

Increasing demand for more powerful and convenient data and informationcommunication has spawned a number of advancements in communicationstechnologies, particularly in wireless communication technologies. Anumber of technologies have been developed to provide the convenience ofwireless communication in a variety of applications, in variouslocations. This proliferation of wireless communication has given riseto a number of manufacturing and operational considerations. In mostcases, however, wireless communications systems have at least one commondenominator—the need or intent to maximize data transfer rates.

Since wireless communications rely on over-the-air (OTA) transmissions,wireless systems and their operation are subjected to a number ofenvironmental interferences, as well as regulatory requirements andrestrictions. These regulatory influences can vary considerably, andeven conflict, across different countries or regions. Wireless devicemanufacturers and service providers often develop industrial standardsto define specific communication schemes, and to help reconcilecompeting or conflicting approaches thereto. Environmental interferencescan vary from naturally occurring phenomena to conflicting wirelesstransmissions. The proliferation of wireless communication has resultedin a number of disparate technologies that may operate in adjacent,partially overlapping, or overlapping frequency ranges or channels.Wireless device manufacturers and service providers must therefore alsocomprehend potential performance and reliability degradations that mayresult from frequency range conflicts.

Among recently emerging communication technologies—especially thosetargeted at or intended for high data transfer rates—variousultra-wideband (UWB) technologies are gaining support and acceptance.UWB technologies are commonly utilized for wireless transmission ofvideo, audio or other high bandwidth data between various devices.Generally, UWB is utilized for short-range radiocommunications—typically data relay between devices within approximately30 feet—although longer-range applications may be developed. Aconventional UWB transmitter generally operates over a very widespectrum of frequencies, several GHz in bandwidth. UWB may be defined asradio technology that has either: 1) a spectrum that occupies bandwidthgreater than 20% of its center frequency; or, as it is more commonlyunderstood, 2) a bandwidth ≧500 MHz.

UWB systems commonly utilize a modulation scheme, known as OrthogonalFrequency Division Multiplexing (OFDM), to organize or allocate datatransmissions across extremely wide bandwidths. OFDM schemes arecommonly utilized, not only in UWB systems, but also in high-bandwidthcommunications systems and protocols such as 802.11(a).

Often, particularly in UWB systems, OFDM schemes are supplemented bydividing a given frequency range into multiple sub-bands. Systems thatutilize these multiple sub-bands in combination with OFDM modulation arecommonly known as Multi-band OFDM. Multi-band OFDM (MBOFDM) in a UWBsystem provides relatively low-power, broad-spectrum communication thatenables high bandwidth data transfer.

Considering UWB as an illustrative example, the Federal CommunicationsCommission (FCC) of the United States has allocated the spectrum from3.1 GHz-10.6 GHz for UWB radio transmissions. This UWB frequencyallocation is unlicensed, leaving the spectrum open to a number ofpotentially conflicting technologies. Due to this unlicensed nature, UWBdevices and systems have to contend with both pre-existing andfuture-developed services that occupy adjacent frequency bands or sharesome portion of the same frequency band. In order to successfullyco-exist, UWB systems should be capable of adapting to certain spectralmasks—selectively limiting transmissions in certain spectral sub-ranges.

Moreover, the relative strength (i.e., power) of UWB signals is alsolimited to a transmit power of −41.25 dBm/MHz. Due to this relativelylow-power, short-range nature of UWB, even a nominal degree of signalfading or interference from an adjacent frequency band can significantlyimpact the signal integrity of a given tone.

In the increasingly common situation where a conventional OFDM system(e.g., a UWB system) must account for one or more spectral maskingrequirements, certain sub-ranges of a frequency band cannot beutilized—decreasing the system's potential data transfer bandwidth. Inorder to achieve a desired high bandwidth data transfer, over anow-limited available sub-portion of a channel, the conventional OFDMsystem has to maximize the raw volume of data transferred over theavailable channel sub-portion. Unfortunately, however, conventional OFDMsystems utilize a number of data coding and redundancy techniques forerror correction and data integrity purposes. Although these techniquesimprove the reliability and integrity of data transmissions, byaccounting or correcting for signal noise or interference, they reduceeffective data transfer bandwidth by significant amounts. Thus, wirelesssystem designers utilizing OFDM techniques may often face a tradeoffbetween achieving optimally high data transfer rates and ensuring dataintegrity or reliability.

As a result, there is a need for a system that provides optimal datathroughput in OFDM-based communication technology while providingreliable data integrity—one that maximizes system utilization of allavailable sub-portions of a given wireless transmission frequencyrange—in an easy, efficient and cost-effective manner.

SUMMARY OF THE INVENTION

The present invention provides a versatile system, comprising variousstructures and methods, for optimizing the utilization, efficiency, andreliability of OFDM-based communications, through selective frequencydomain transformation prior to transmission. The system of the presentinvention provides a frequency domain transformation for a group ofsignal sub-carriers, prior to transmission, in a manner that effectivelyspreads sub-carrier energy across a group of sub-carriers. The presentinvention is readily adaptable to a number of design requirements andvariables, and may be implemented in a variety of OFDM-based systems.The present invention thus provides optimal data throughput inOFDM-based communications in an easy, cost-effective manner.

The system of the present invention provides for selective spreading ofcarrier data across multiple carrier paths within an OrthogonalFrequency Division Multiplexing (OFDM) system, particularly anultra-wideband system. The present invention receives data from a datainput, which passes to a scrambler (or randomizer) function. The datathen passes to a convolutional code function, the output of which ispunctured by puncturing function. An interleaver function receives thepunctured code data, and cooperatively operates with a mapper element toprepare the punctured code data for pre-transmission conversion by anIFFT. The mapper element comprises a dual carrier modulation function.The dual carrier modulation function associates two punctured code dataelements and transforms those elements into a format for transmission ontwo separate signal tones.

More specifically, certain embodiments of the present invention providean ultra-wideband wireless communications system having a data input,and a coding function adapted to receive and code the data. Aninterleaver function is adapted to receive coded data from the codingfunction, and to cooperate with a mapper element. The mapper element hasa dual carrier modulation function. A pre-transmission conversionfunction receives coded data from the mapper element, and convert thecoded data prior to wireless transmission of the data.

Other embodiments of the present invention provide an OFDM-based datatransmission system. The OFDM system has a data input, and a codingfunction adapted to receive and code the data. A dual carrier modulationfunction is provided, and adapted to selectively transform the codeddata. A pre-transmission conversion function receives the coded data,after transform by the dual carrier modulation function, and convertsthe coded data prior to its transmission.

Certain embodiments of the present invention further provide a method ofmodulating data within an OFDM-based communications system. An indexseparation factor is provided or determined. A first data element,having a first index value, is associated with a second data element,having a second index value—separated from the first index value by theseparation factor. A column vector is formed having [first data element;second data element]. A transformation matrix is provided, and appliedto the column vector, to render a transformed column vector. Thetransformed column vector is thereafter transmitted at tonescorresponding to the first and second index values.

Other features and advantages of the present invention will be apparentto those of ordinary skill in the art upon reference to the followingdetailed description taken in conjunction with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the invention, and to show by way ofexample how the same may be carried into effect, reference is now madeto the detailed description of the invention along with the accompanyingfigures in which corresponding numerals in the different figures referto corresponding parts and in which:

FIG. 1 provides an illustration depicting one embodiment of an MBOFDMUWB system segment; and

FIG. 2 provides an illustration depicting an embodiment of an MBOFDM UWBsystem segment in accordance with certain aspects of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

While the making and using of various embodiments of the presentinvention are discussed in detail below, it should be appreciated thatthe present invention provides many applicable inventive concepts, whichcan be embodied in a wide variety of specific contexts. The presentinvention is hereafter illustratively described primarily in conjunctionwith the design and operation of an ultra-wideband (UWB) communicationssystem utilizing an Orthogonal Frequency Division Multiplexing (OFDM)scheme. Certain aspects of the present invention are further detailed inrelation to design and operation of a Multi-band OFDM (MBOFDM) UWBcommunications system. Although described in relation to such constructsand operations, the teachings and embodiments of the present inventionmay be beneficially implemented with a variety of data transmission orcommunication systems or protocols (e.g., IEEE 802.11(a)), dependingupon the specific needs or requirements of such systems. The specificembodiments discussed herein are, therefore, merely demonstrative ofspecific ways to make and use the invention and do not limit the scopeof the invention.

The present invention provides a versatile system of structures andmethods that optimize the utilization, efficiency, and reliability of anOFDM-based communications system. The present invention provides thisoptimization through selective frequency domain transformation of agroup of signal sub-carriers prior to transmission. The system of thepresent invention effectively spreads sub-carrier energy across a groupof sub-carriers—mitigating signal diversity losses and improving systemperformance at high coding rates. The present invention is readilyadaptable to a number of design requirements and variables, and may beimplemented in a variety of OFDM-based systems. The present inventionthus provides optimal data throughput in OFDM-based communications in aneasy and efficient manner.

OFDM-based wireless communication systems commonly utilize apre-transmission conversion function to convert a data signal from thefrequency domain into the time domain, for OTA transmission over awireless channel. During transmission over the wireless channel, somedegree of signal noise (e.g., interference) is added to the time domaindata signal. As the time domain signal is received, a post-transmissionconversion function is utilized to convert the signal back into thefrequency domain, for subsequent signal processing or communication.Often, such pre-transmission and post-transmission conversion functionstake the form of Inverse Fast Fourier Transforms (IFFTs) and FastFourier Transforms (FFTs), respectively.

Within the context of an OFDM-based UWB system, a pre-transmission IFFTcommonly has 128 points (or tones). Depending upon the type ofcommunications system, or specific design or performance requirements,however, an IFFT may have any desired or required number of tones. Inmost embodiments, one hundred of those tones are used as data carriers,twelve are pilot carriers (i.e., carry data known to receiver that ituses to ensure coherent detection), ten are guard carriers, and six arenull tones. The ten guard carriers may be configured to serve a numberof concurrent or independent functions. For example, some portion of theguard tones may be configured to improve signal-to-noise ratios (SNRs),by loading those guard carriers with critical data (e.g., unreliabledata) for redundant transmission. Some portion of the guard tones may beconfigured (e.g., left unutilized) as frequency guard bands, to preventinterference to or from adjacent frequency bands. Of the six null tones,one typically occupies the middle of the available signal spectrum(i.e., dc tone), and the others may be selectively configured ordesignated to conform to a desired spectral mask (e.g., UWB, 802.11,802.16).

Within a MBOFDM system, data tones across different bands are typicallyloaded with quadrature phase-shift key (QPSK) data. For ahigh-throughput MBOFDM system, there are a number of techniques that maybe used to manipulate or tailor system data rates. Typically, suchsystems employ some sort of convolutional code for errordetection/correction purposes. For example, in a UWB MBOFDM system, an[R=⅓, k=7] convolutional code may be provided as a forward errorcorrection (FEC) code. Such codes can be manipulated by variouspuncturing schemes to achieve a desired data rate (e.g., [R=¾, k=7] for480 Mbps). In addition to code puncturing, techniques such as frequencydomain spreading and time domain spreading may be employed to dividedown to a desired data rate.

For purposes of background and explaining certain aspects of the presentinvention, reference is made now to FIG. 1, which provides a high levelfunctional depiction of a transmission segment 100 of an MBOFDM UWBsystem. Data 102 is delivered to a scrambler function 104, in bitformat. Function 104 operates on the data bits to approximate arandomization thereof. That data is then transferred to a convolutionalcode function 106, which converts the data into some desired codeform—an [R=⅓, k=7] code form, for example.

The coded data is next transferred to a puncturing function 108 toreduce the coded bits, by some puncturing characteristic, to a desiredpunctured code equivalent. For example, if five data bits are convertedinto an [R=⅓, k=7] code form, fifteen coded data bits are transferredfrom function 106 to function 108. If a desired punctured codeequivalent is ¾, then function 108 reduces the fifteen data bits,delivered from function 106, by a puncturing characteristic of eightbits, and outputs 7 coded bits. Depending upon system requirements, theconvolutional code form of function 106 and the puncturingcharacteristic of function 108 may be varied individually orcooperatively to render a desired punctured code equivalent (e.g., ½,¾). Coded bits are output from function 108 to a data mapping function110. Function 110 performs various arranging and mapping functions onthe coded data prior to loading that data to pre-transmission conversionfunction 112. Function 110 may perform an interleaving function to loadcoded data bits to carrier tones such that adjacent coded bits are noton adjacent carrier tones. Function 110 may also convert the coded databits to a QPSK format, and group the QPSK data according to the numberof available data carrier tones (e.g., 100). In this instance, function112 is an IFFT that converts the coded data from the frequency domain tothe time domain, prior to transmission over a wireless channel.

Utilizing such an approach, a conventional MBOFDM UWB system at highdata rates may experience increased bit error ratios (BER) or parityerror ratios (PER). Since data is transferred over multiple bands,exposure to varying SNRs is increased. Depending upon the magnitude ofthe packet sizes transferred, and the SNR values experienced, BERs orPERs can increase significantly—representing a significant loss ofperformance in the system. Such losses, coupled with the performancelosses introduced by techniques such as puncturing, significantly reducethe overall system efficiency.

Referring now to FIG. 2, certain aspects of the present invention areexplained in relation to a high level functional depiction of atransmission segment 200 of an MBOFDM UWB system according to thepresent invention. Data 202 is delivered to a scrambler function 204, inbit format. Function 204 operates on the data bits to approximate arandomization thereof. That data is then transferred to a convolutionalcode function 206, which converts the data into some desired codeform—an [R=⅓, k=7] code form, for example.

The coded data is next transferred to a puncturing function 208 toreduce the coded bits, by some puncturing characteristic, to a desiredpunctured code equivalent. For example, if five data bits are convertedinto an [R=⅓, k=7] code form, fifteen coded data bits are transferredfrom function 206 to function 208. If a desired punctured codeequivalent is ¾, then function 208 reduces the fifteen data bits,delivered from function 206, by a puncturing characteristic of eightbits, and outputs 7 coded bits. Depending upon system requirements, theconvolutional code form of function 206 and the puncturingcharacteristic of function 208 may be varied individually orcooperatively to render a desired punctured code equivalent (e.g., ½,¾).

Coded bits are output from function 208 to an interleaver function 210.Function 210 cooperates with a mapping function 212 and a groupingfunction 214. Function 210 is configured to load coded data bits tocarrier tones within available sub-bands such that adjacent coded bitsare not on adjacent carrier tones. Function 212 converts the coded databits to a QPSK format. Grouping function 214 may utilize either afrequency domain spreading function or a time domain spreading function,in addition to a dual carrier modulation (DCM) function 216, to sort thecoded QPSK data for loading on the carrier tones (e.g., 100 tones).Functions 212, 214 and 216 are implemented within a single operationalmapping element 218. Element 218 may be implemented as a functional partof interleaver 210, or as a separate functional element. Once functions210-216 have configured coded data in a desired arrangement, that datais then loaded to pre-transmission conversion function 220. In thisembodiment, function 220 is an IFFT that converts coded data from thefrequency domain to the time domain, prior to transmission over awireless channel.

Dual carrier modulation function 216 provides a transform performed uponthe QPSK data that optimizes data spreading over the tones, so as tooptimize the likelihood of successful data recovery on at the receiver.For a given number of carrier tones (e.g., 100), function 216 parses thedata within a single symbol and associates a given data element (s₁)with another data element (s_(A)) separated from (s₁) by some desiredseparation factor (α). The separation factor may be selected or adjustedto account for a number of systems variables, such as tone fadingcharacteristics for a particular wireless system. In a 100 carrier toneapplication, for example, a separation factor of 2 may be desired, toprovide the maximum possible separation between (s₁) and (s_(A)). Thevalue of A is determined by:

$\begin{matrix}{A = {\frac{{{No}.\mspace{14mu} {of}}\mspace{14mu} {tones}}{\alpha} + 1}} & (1)\end{matrix}$

Thus, for an application having 100 sub-carriers and a desiredseparation factor of 2, (s₁) is associated with (s₅₁), (s₂) isassociated with (s₅₂), etc. These associated pairs of data elements arerendered as data symbols of the form of column vectors [s₁; s₅₁], [s₂;s₅₂] . . . [s_(K); s_(L)]; where each data symbol is transmitted on boththe (K) and (L) tones.

Thus, a 2 sub-carrier group has a pre-transmission data symbol, to betransmitted in the sub-carriers indexed as K and L, in the form of acolumn vector [SK;SL]. Transmission of this data could take the form ofmapping [SK;SL] to sub-carriers K and L, applying an IFFT, adding eithera zero-padded or a cyclic prefix, and then transmitting the resultingtime-domain OFDM symbol. According to the present invention, however, aDCM transform (T) is first applied to [SK;SL], rendering a transformedsymbol [YK;YL]—where [YK;YL]=T*[SK;SL]. T is an N×N transformationmatrix, which may be provided as a wide variety of transform types. Inthis example, T may be chosen as a 2×2 orthogonal transform.

For this example, then, [YK;YL] is loaded to an IFFT, and transmittedover the wireless channel. On the receiving end of such a system, asignal received undergoes a number of routine operations, such assynchronization, cyclic prefix removal, and post-transmission conversionby an FFT. The received data symbol is now in the format of [RK;RL].[RK;RL]=[HK*YK;HL*YL], which is equivalent to [HK, 0;0, HL]*[YK;YL].[HK, 0;0, HL] represents some additive or transformative characteristicof the transmission channel (e.g., noise effects) that may be determinedor estimated by, for example, transmitting a training information setknown at the receiver. [HK, 0;0, HL]*[YK;YL]=[HK, 0;0, HL]*T*[SK;SL];from which [SK;SL] may be extracted. The effective channel (H_(e)) atthe receiver is [HK, 0;0, HL]*T. H_(e) may be removed by either usingstandard inversion (ZF), an LMMSE-type channel compensation (which alsoaccounts for noise variances on the two sub-carriers), or by using aMAP/ML decoder. In various embodiments, sub-carriers may be derived froma plurality of OFDM symbols. In various embodiments, the DCM transform(T) may be varied from one sub-carrier group to another, as well.

As previously noted, (T) may be easily extended to an N×N case, as anN×N matrix. In certain embodiments, for example, having an N×Ntransform, (M) sub-carriers in an OFDM symbol are divided into (M/N)groups of (N) sub-carriers. The transform (T) is applied to each ofthese sub-carrier groups. Different transforms (T1), (T2) . . . (T_(X))may be applied to each of the (M/N) groups of sub-carriers, dependingupon specific operational requirements or characteristics. In otherembodiments, (M) sub-carriers may be divided into groups of (N₁), (N₂) .. . (N_(P)) sub-carriers, where (N_(P))—the size of the sub-carriergrouping—is varied within an OFDM symbol, or between OFDM symbols.Different transforms (T1), (T2) . . . (T_(P)) may be applied to eachgroup of sub-carriers, respectively. The DCM transforms may also beapplied to any combination of data, pilot or null tones. The size andextract structure of transform (T) may also be time-varied within agiven packet—i.e., between symbols—as well as across packets.

In certain embodiments of the present invention, where a MBOFDM systemconsists of 100 data tones in each OFDM symbol, (T) may be provided inone of several useful formats. As noted in the example described above,T=[2 1;1 −2] may be used in embodiments where each sub-carrier groupingconsists of data tones at location K and K+50—i.e., where pairedsub-carriers are separated by 50 tone locations prior to the insertionof a pilot tone. In other embodiments (T) may take the form [1 1;1 −1],[2 1;−1 2], or [cos θ sin θ; −sin θ cos θ], where 0 takes some valuebetween 0 and π/2. The transform (T) may also be a complex valuedtransform, such as [1 exp(jθ); exp(−jθ) 1], where θ takes some valuebetween 0 and 2π. In various other embodiments, transform (T) may beprovided or modified in a number of ways, such as interchanging rows orcolumns, negating a row or column, and conjugating a row or column. Theselected format of (T) may be determined based on a number offactors—such as overall transmission efficiency or decoding schemecomplexity—or various tradeoffs therebetween.

In certain alternative embodiments of the present invention, symbols ofa sub-group may be “spread” by mapping each column vector to different,higher-order constellations. For example, in one embodiment, 2 symbolsmay be grouped and “spread” over 2 sub-carriers. In this embodiment,[SK; SL] is a symbol vector corresponding to tone indices K and L. C isa constellation, of cardinality (A), used by symbols SK and SL. In thisembodiment, a MBOFDM system, constellation C is QPSK, with a cardinalityof A=4. Symbol vector [SK; SL] is mapped to a new vector [YK; YL], wherecomponents YK and YL are derived from constellations C1 and C2,respectively. Constellations C1 and C2 have cardinalities of (A1) and(A2), respectively. (A1) and (A2) may be chosen such that they arelarger than (A) and <(A)². For example, a MBOFDM system, (A1) and (A2)may be chosen to be 16, and constellation C1 and C2 may be chosen to be16 QAM—having different mappings of [SK; SL] to C1 and C2.

Thus, by the present invention, the pre-transmission transform (T)provides an additional spreading domain. This provides an efficientsub-carrier spreading scheme for OFDM-based application—one that isreadily adaptable to a wide variety of wireless communicationapplications. Data is efficiently paired, or otherwise grouped, fortransmission on multiple sub-carriers, having a desired spread acrossthe channel spectrum. This spreading decreases data errors due to lossor interferences at specific tones—significantly improving overall dataaccuracy and reliability. Moreover, a system according to the presentinvention may be adapted to account for varying channel conditions (e.g.interference spikes). Where sufficient transmitter or receivertechnology exists, feedback and control systems may be implemented todynamically change transform (T) as transmission conditions change.Additionally, transform (T) may be provided to optimize systemperformance for a particular parameter (e.g., PER, BER, SNR).

A number of variations are comprehended by the present invention. Theembodiments and examples set forth herein are therefore presented tobest explain the present invention and its practical application, and tothereby enable those skilled in the art to make and utilize theinvention. However, those skilled in the art will recognize that theforegoing description and examples have been presented for the purposeof illustration and example only. For example, the system of the presentinvention may be effectively applied to single or multiple antennasystems. In other variations, the pre-transmission transform may beprovided as a unitary transform matrix or an orthogonal transformmatrix. The teachings and principles of the present invention areapplicable or adaptable to wide variety of communications protocols(e.g., UWB, 802.11, 802.16). The description as set forth herein istherefore not intended to be exhaustive or to limit the invention to theprecise form disclosed. As stated throughout, many modifications andvariations are possible in light of the above teaching without departingfrom the spirit and scope of the following claims.

1. A method of modulating data within an OFDM-based communicationssystem, the method comprising the steps of: providing an indexseparation factor; associating a first data element, having a firstindex value, with a second data element, having a second index valueseparated from the first index value by the separation factor; forming acolumn vector of having a form of [first data element; second dataelement]; providing a transformation matrix; applying the transformationmatrix to the column vector to render a transformed column vector; andtransmitting the transformed column vector at tones corresponding to thefirst and second index values.